With the known detectors of this type, the output signal of the microphone is first of all amplified, then, generally speaking, compared with a set reference voltage in a comparator, the output of which can have two possible states according to the relative value of the signal coming from the microphone and the reference voltage.
These detectors activate the alarm when affected by an aperiodic compression wave, while they do not react to a periodic signal such as an audible sound, the monitoring being carried out especially on the shape and size of the signals picked up.
With these known differential detectors, as with previous state of the art equipment designed to warn of untimely openings of doors and windows in closed premisses, the threshold level must be set manually, case by case.
This setting is intricately linked, in practice, with any faults in the sealing of the site in question which might occur, and also with the extreme flexibility of some building materials used, which, in the case of a strong wind, give rise, due to a pushing effect or infiltration, to pressure variation within the premisses.
So as to avoid all risks of activating the alarm for reasons other than breaking and entering, the threshold level of these detectors should be set at a relatively high level, so that they do not take into account atmospheric disturbance which while being passing and random, is yet inevitable since it is conditioned by the presence of a strong wind. This setting is detrimental to the efficiency of the detector in calm weather.